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The Effect of Conductive Ink Layer Thickness on the Functioning of Printed UHF RFID Antennas The authors of this paper investigate the effect of conductive ink layer thickness on performance of RFID antennas, and ways of optimizing tradeoffs between performance characteristics and ink thickness. ¨ rninen, Antti Vuorima¨ ki, Leena Ukkonen, By Sari Linnea Merilampi, Toni Bjo Pekka Ruuskanen, and Lauri Syda¨ nheimo

ABSTRACT | In this study, the effect of the conductive ink layer

KEYWORDS

thickness on the performance of printed ultra-high-frequency

printed electronics; screen printing; ultra-high-frequency

|

(UHF) radio-frequency identification (RFID) tag antennas was

(UHF) RFID tag

Passive radio-frequency identification (RFID);

investigated. A simple quarter wave dipole tag for European UHF RFID frequencies was designed to be tested in this study. All the tags were made by using screen-printing technique. Three different thicknesses for the ink layer were used. Performance of the tags was analyzed by the measurement of threshold and backscatter power. The results show that it is possible to produce RFID tag antennas by screen printing and it is possible to optimize the tag performance by adjusting the thickness of the electrically conductive layer. The results show how the performance characteristics deteriorated when the thickness of the printed ink layer was reduced. However, it was also shown that thin ink layers can be used in some applications and cost savings can be achieved in this way. It is therefore important to recognize these effects on the performance.

Manuscript received October 27, 2008; revised April 12, 2010; accepted May 5, 2010. Date of publication June 28, 2010; date of current version August 20, 2010. This work was supported by the Finnish Cultural Foundation-Satakunta Regional Fund and the ¨ rninen was supported by TISE Graduate Ulla Tuominen foundation. The work of T. Bjo School and TEKES (the Finnish Funding Agency for Technology and Innovation). ¨ ki, and P. Ruuskanen are with the Department of S. L. Merilampi, A. Vuorima Electronics, Tampere University of Technology, Pori, Electronics, 28100 Pori, Finland (e-mail: [email protected]; [email protected]; [email protected]). ¨ rninen, L. Ukkonen, and L. Syda ¨ nheimo are with the Department of T. Bjo Electronics, Rauma Research Unit, Tampere University of Technology, 26100 Rauma, Finland (e-mail: [email protected]; [email protected]; [email protected]). Digital Object Identifier: 10.1109/JPROC.2010.2050570

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I . INTRODUCTION The integration of electronics, for instance, radio-frequency identification (RFID) tags, in different applications and products such as packaging, machines, clothing, etc., is becoming more and more important in order to gain greater functionality in the products. With traditional electronic manufacturing techniques the integration may not be technically and economically competitive in mass production. Etching is typically used in tag manufacturing to produce the conductive antenna pattern. Although etching is a well-known and efficient process, it has a few drawbacks. The etching process contains many process phases and uses different chemicals which is not environmentally friendly and adds costs. In etching, the unwanted metal is removed from the substrate, which creates material loss and produces waste. Substrate must also tolerate the process chemicals, which reduces the substrate selection. One trend of today is to embed electronics into other structures. In case of RFID tags, it would be convenient to use the item which is to be identified as the substrate. This would eliminate the need for additional substrate material and create cost savings. For these reasons additive processes are gaining interest. Printing methods are the most common additive methods applied in RFID tag antenna fabrication and they are currently under wide research. In

Proceedings of the IEEE | Vol. 98, No. 9, September 2010

0018-9219/$26.00 Ó 2010 IEEE

Merilampi et al.: The Effect of Conductive Ink Layer Thickness on the Functioning of Printed UHF RFID Antennas

addition to printing methods, other novel additive fabrication methods are under development. Dry phase patterning (DPP), developed by Acreo AB (Kista, Sweden), is a method for patterning metal layers on flexible substrates like plastics and paper [1]. In the DPP method, the pattern definition and the material removal are done in the same step. The method is environmentally friendly and high process speed has been demonstrated. Printing is a simple and fast method which also enables mass production. The printing process contains two process steps: printing and curing. The material loss and the use of different chemicals are considerably lesser in comparison with etching. Although the printing is a promising tag antenna manufacturing method, there is lack of information about the tag materials and the performance of printed tags. Recently, different printing techniques have been adapted for electronics manufacturing. For example, screen printing, gravure printing, flexography, and ink jet printing seem to be promising techniques for printable electronics [2]–[6]. Printing makes it possible to integrate electronic circuits on differently shaped surfaces and on new types of flexible and stretchable printed circuit board (PCB) materials. An important part of any electrical circuit is the electronically conductive pattern. When the patterns are produced by printing techniques several important competitive factors have to be taken into account. The patterns are usually made using silver particles as the conductive medium. To minimize the use of the costly silver the patterns should therefore be as thin and as narrow as possible. However, the pattern must be thick enough to guarantee good conductivity and low ohmic losses. This is very important especially at higher frequencies. It therefore becomes crucial to consider what thickness is sufficient to achieve adequate performance together with low unit cost [7], [8]. The different prices of inks for different printing techniques must also be taken into account. Different printing techniques also have different limitations [6]. The motivation for this work was to investigate the optimum thickness of the printed, electrically conductive silver layer, for ultra-high-frequency (UHF) RFID tags. The same kind of investigation was done in [8]. According to [8], the total amount of ink affects the radiation efficiency of a printed tag antenna. The tag antenna performance of silver ink antennas was compared with copper antennas in [9]. Nikitin et al. [9] suggested that with proper design it is possible to make very good silver ink tags. At high frequencies, like UHF, the current density is packed into the region near the surface of a good conductor. This is called the skin effect. Skin depth or penetration depth is defined as the depth below the surface of the conductor at which the amplitude of an incident electric field E has decreased by factor 1/e (approximately 0.37) [10]. An approximate expression for the skin depth can be found by considering the normal incidence of a plane wave

into a good conductor. Suppose that the electric field oscillates along x-axis and the wave propagates in the positive z-direction. Then, the incident field is defined by ~ Eðz; tÞ ¼ x^Re EðzÞej!t

(1)

where x^ is a unit vector in x-direction, j is the imaginary unit, ! is the angular frequency, and t is the time. In a lossy medium, characterized by constitutive parameters permittivity, permeability, and conductivity ð"; ; Þ, the phasor of the incident field is EðzÞ ¼ Eð0Þez

(2)

where Eð0Þ is the amplitude of the wave at the surface of the medium and is the propagation constant given by

¼

qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi j!ð þ j!"Þ:

(3)

In a good conductor, the real part of , which models the attenuation of the wave, can be approximated by [10]

ReðÞ ¼

rffiffiffiffiffiffiffiffiffi ! 2

(4)

where is the attenuation constant. Then, applying the definition of skin depth, one can write e1 Eð0Þ ¼ jEðÞj ¼ jEð0Þjje j

(5)

where denotes the skin depth and then solve

¼

1 1 ¼ pffiffiffiffiffiffiffiffiffiffi f

(6)

where f is the frequency, is the magnetic permeability ð0 Þ, and is the electric conductivity of the conductor material (ink in our study). The magnitude of current density J in the conductor at depth z, due to the incident field, is now JðzÞ ¼ jEðzÞj ¼ Eð0Þez ¼ Eð0Þez= :

(7)

According to (6), the penetration depth is inversely proportional to the frequency and therefore the conductive

Vol. 98, No. 9, September 2010 | Proceedings of the IEEE

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faces at z ¼ 0 and z ¼ t. Then, the power loss P0 can be further approximated as

P0 ¼ 2

Zt=2ZwZ l

2 Vrms 1 e2z dx dy dz: l2

(11)

0 0 0

Fig. 1. Rectangular conducting structure in Cartesian coordinates.

(ink) layer does not have to be very thick in UHF (300 MHz–3 GHz) applications. With different printing techniques, it is possible to produce ink layers, which have a thickness of the same order of magnitude as the skin depth while the resistance of the layer remains low [2]–[7]. If the thickness of the ink layer is reduced, the ohmic losses increase, because the structure begins to disturb the current flow. Increased losses in the antenna structure decrease the radiation efficiency and therefore the ink layer must not be made too thin. The effect of the characteristics of the conductor material on losses can be evaluated as follows. Consider a rectangular conductor shown in Fig. 1. The power loss in the structure can be approximated using Joule’s law as

P¼

Z

~ J~ E dv

Vrms l

P0 ¼

Gs ¼

P¼

2 Vrms 1 e2z dx dy dz: l2

Zt

ez dz ¼

ð1 et Þ:

(13)

0

1 1 ð1 et Þ ¼ ð1 et= Þ :

(14)

The resistance of the rectangular conductor in Fig. 1 can be thought as a parallel resistance of the sheet resistance of the upper layer RU (thickness t=2) and lower layer RL (thickness t=2), because t w and the only significant faces are the faces at z ¼ 0 and z ¼ t. The expression of the resistance can then be written as l 1 1 1 l 1 ð1 et=2 Þ : þ ¼ w RU R L 2w

(15)

Considering (15) and Ohm’s law, the expression for the loss power from (12) in terms of the effective current is

P0 ¼

0 0 0

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dGs ¼

Rs ¼

(10)

Current flows in all surfaces of the structure (Fig. 1), but if t w, the only significant contribution comes from the

(12)

1 Recalling that the sheet resistance Rs ¼ G1 s and ¼ , the sheet resistance of the conductive layer becomes

R¼

Z tZwZ l

Zt 0

(9)

where Vrms is the root mean square value of the voltage and l is the length in Fig. 1. Taking this into account the power loss becomes

2 Vrms w ð1 et= Þ: l

Considering (7), the sheet conductance Gs of conductive layer of thickness t can be estimated as follows:

(8)

where P is the power loss, ~ J is the current density, and ~ E is the electric field [11]. The expression for the electric field strength inside the conductor is given in (2) and its value at the surface of the conductor is, in this case

Eð0Þ ¼

Recalling that conductivity is the inverse of resistivity ð ¼ 1 Þ, the expression for loss power becomes

I2rms l 2 ð1 et= Þð1 et=2 Þ 4w

(16)

where Irms is the root-mean-square value of the current. Equation (16) implies that as conductor thickness tends to



zero, the power loss increases as the inverse of the thickness. When t ; 4 t I2rms l 0 : lim P ¼ lim P ¼ wt

ð1 et= Þð1 et=2 Þ and

2

Fig. 2. Tag design.

On the whole, the presented formulation to model the losses is a rough estimate because the geometry is neglected. Nevertheless, it gives an idea of how the thickness of the conductor material affects the power loss. In an antenna structure, the power loss is connected to the radiation characteristics through the radiation efficiency er , given by er ¼

Pr Rr ¼ PL þ Pr RL þ Rr

(17)

where subscript L refers to the combined conductor and dielectric loss power and corresponding loss resistance and r refers to the radiated power and radiation resistance.

II . EXPE RIMENTAL ARRANGEMENTS A. Printing Screen-printing technique was used to produce prototype RFID antennas. Polyethyleneterephtalate (PET) was selected as the substrate material. The thickness of the substrate PET foil is 75 m and the relative permittivity is about 3. The surface roughness ðRz Þ of the PET foil was measured using profilometer. The surface roughness value Rz is the average of the five highest points and the five lowest points of the surface. The value of Rz for the PET foil was 2.2 m. The characteristics of the conductive silver ink are presented in Table 1.

The tags were produced using three different thicknesses. One thickness corresponds to the penetration depth calculated on the basis of information obtained from ink manufacturers’ datasheet. The ink layer of the other two tags was thicker than the penetration depth. Two different screens were used to manufacture these tags. The descriptions of the samples and the screens which were used in printing are shown in Table 2. Two layers of ink were used for two of the samples. The first ink layer was cured before printing the second layer. The ink must be cured to evaporate the solvents and thus to form solid ink film with feasible conductivity.

B. Tag Design and Simulations The tag antenna for this experiment is a rectangular short dipole shown in Fig. 2 The dimensions of the tag are L ¼ 97 mm, W ¼ 8 mm, s ¼ 17 mm, t ¼ 0.5 mm, u ¼ 2 mm, and v ¼ 5 mm. Design was accomplished using Ansoft’s high-frequency structure simulator (HFSS) to determine the dimensions for the matching loop to tune the antenna for an operating frequency of 866 MHz. In the simulations, the ink thickness was set to 40 m. The IC-chip for this design is Alien H2.

III . MEASUREMENTS The thicknesses of the antennas were measured using software connected to an optical microscope. Cross sections were made from the samples and the thickness of the ink layer was measured at 20 different places. The average

Table 1 Characteristics of the Conductive Silver Ink

Table 2 Descriptions of the Samples


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thickness and standard deviation was calculated from these values. Two different measurements were conducted to find out the performance of the tags. The first one of these is the threshold measurement where, at each frequency of interest, the transmitted power is increased until the tag responds to the reader’s query command. The measured transmitted threshold power is then used to derive the power on tag, which is the transmitted threshold power normalized by the pathloss from transmitting antenna to the tag and the gain of the transmitting antenna. The pathloss we use for normalization is provided by the calibration procedure of the used equipment. The normalization of the transmitted threshold power enables us to remove the effect of multipath propagation from the results and thus compare the tag antennas consistently. In addition, as the only difference in the geometry of the measured tags is the conductor thickness, the radiation patterns are expected to be identical and thus the measured power on tag is directly proportional to the radiation efficiency and power transfer between the tag antenna and the IC. As the minimum power level at which the IC is activated depends strongly on the power reflection coefficient (PRC) [12] between the tag antenna and the IC, by studying the power on tag as a function of frequency, optimal operation frequency at each of the measured tags can be identified. In addition to impedance matching, backscatter properties are also a significant factor in the performance of an RFID tag. The radar cross section (RCS) of the tag in the reflective modulation state determines how well the tag can reflect power back to the reader and the difference between the reflectivity of this state and the nonreflective modulation state determines the power of the modulated backscattered signal [13]. This power was the other measured quantity. Both of these measurements were performed using a Voyantic Tagformance measurement system [14].

IV. RESULTS AND DI SCUS SI ON A. Morphology of the Conductive Layers The measurement statistics (average thickness, standard deviation, and minimum and maximum values) of the ink layer thickness measurements are presented in Table 3. The thinnest conductive layer was 13.9 m and thickest one was 50.5 m. Screen printing is the only printing Table 3 Measurement Statistics of the Thickness of the Test Samples

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Fig. 3. Scanning electron microscope micrograph of sample 1, average thickness 13.9 m.



method which allows the printing of this thick ink layers by a single print press. Typical ink film thickness printed with gravure printing or flexography films is G 10 m, and with offset lithography the ink film thickness is even smaller (1–2 m) [15]. With these printing methods, it is even more important to pay attention to tag performance, print quality, and ink properties. Figs. 3–5 show the cross-section micrographs of the three conductive layers. As shown in the figures, the ink layer is not uniform and thickness variations occur even when the substrate is smooth. Waving of the ink layer is typical of screen printing, because the ink is pressed through a mesh. The layer is thinner in the areas where there are threads during printing. In case of sample 3, the standard deviation is large, since the thread of the screen is thicker than in case of printing samples 1 and 2 and the screen is less dense (see Table 2). However, according to [8], the nonuniform thickness may not be an issue. The total amount of ink is probably more important. Though, it was found in [16] that if the conductive layer is uneven in high current density regions, this makes the tag performance worse. In [17], it was stated



that the higher the vehicle (binder polymer and solvents) content of the ink, the more uneven the ink layer surface printed with the offset lithography method. The uneven ink film surface is also noticed in [16] with patterns printed with the gravure method and in [8] with flexography. So, uneven ink layers are also common for other printing methods than screen printing. This is why the effect of fluctuating ink surface on the electrical performance is important to examine. Our future work is to investigate the losses of a smooth ink film compared with a waving one. The silver ink of this study is polymer thick film ink. It consists of silver flakes, polyester resin (binder material), solvents, and additives [18]. Each component has a purpose that is essential for the overall performance of the ink. The ingredients are selected depending on the printing method. In this study, only screen printable tags are discussed although the ink composition of other printing methods excluding inkjet inks is of the same kind [17]–[19]. In the ink jet method, the ink does not contain a binder polymer and the particles are smaller (nanoparticles). Nanoparticles are required to ensure the sufficient conductivity of the ink layer, since the ink film is thin (1 m). Small particle size is also required to prevent the clogging of the small ink jet nozzles. Due to the nanoparticles, a dispersing agent is needed to maintain the evenly dispersing of nanoparticles and to prevent the agglomeration of the particles. After printing, the ink requires sintering to remove the dispersing agent and to fuse the particles and thus forming a conducting ink layer. If the sintering is performed by heat,

the substrate must tolerate higher than 200 C temperatures and thus this reduces the substrate selection [20]. In case of other printing methods, the curing temperatures are lower (100 C–200 C) [17]–[19]. Ink jet is suitable for flexible manufacturing where every product may be different. It is less suitable in mass production. It is particularly useful in printing small accurate structures.

B. Electrical Performance The penetration depth of the ink layer was calculated from (6) using the following parameters: f ¼ 866 MHz, 0 ¼ 4 107 Vs/Am, ¼ 1.25 MS/m. The calculated penetration depth is about 15 m at 866 MHz. The attenuation of the current density as a function of penetration depth was calculated using (7). The current density decreases to approximately 63% of its initial value at the surface at one penetration depth and to 86% and 95% at two and three penetration depths, respectively. Therefore, in terms of current packing, using much thicker conducting layer than three penetration depths would not help much, as only 5% of the total current would penetrate beyond the three penetration depths in the conductor. On the other hand, below we demonstrate through simulations and measurements that for RFID tag antennas also using thinner conducting layer can provide an advantageous tradeoff between the electrical performance and material costs. In Fig. 6, the measured and simulated threshold power curves with different conductive layer thicknesses are presented as a function of frequency. From these results, it

Fig. 6. Measured threshold powers and simulated PRCs as a function of frequency.


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can be seen that the shape of the measured threshold power curves matches with that of the simulated PRCs. This is explained by the fact that the impedance of a resonant antenna has strong frequency dependency whereas the gain of these antennas exhibit weaker frequency dependency within the studied frequencies. As shown in Fig. 6, in our case, decrease in conductor thickness shifts the operating frequency downwards. This is because the thickness of the ink layer affects the antenna input impedance, but as it depends on the antenna geometry as well as on the IC’s input impedance, general conclusions about the direction of this shift cannot be made. Nonetheless, it can be observed that the frequency shift per percentage change in the conductor thickness is greater between 50.5 and 21.5 m than between 21.5 and 13.9 m. This can be due to the nonlinear relationship between the loss resistance of the antenna and conductor thickness suggested in (15). Equation (16) suggests that the power loss depends on the dimensions of the conductive layer. In particular, as the thickness of the layer decreases, the power loss increases. To verify the thickness dependence proposed by (16), the antennas were simulated first in free space, to isolate the loss resistance due to the conductor. After this, the simulation was repeated for the conductive layer on the substrate, in order to take into account the contribution of the dielectric loss resistance arising from the substrate material. Simulation results presented in Fig. 7 predict the loss resistance to increase for each studied layer thickness as a function of frequency and that the difference between the conductor loss resistance of the thinnest (13.9 m) and thickest (51.5 m) conducting layers is approximately 5 . Also for each studied conductor thickness, adding the substrate increases the loss resistance, as expected, since the substrate introduces additional dielectric loss to the structure. Also, the relative change of the loss resistance due to the added substrate seems to remain approximately constant 9% for the studied conductor thicknesses. Equation (17) describes the radiation efficiency and Fig. 8 illustrates the simulated frequency behavior of the radiation efficiencies of the manufactured tags. For all

Fig. 7. Simulated loss resistances.

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Fig. 8. Simulated radiation efficiencies.

conductor thicknesses, whether the tag is placed on the substrate, the simulated radiation efficiencies increase as a function of frequency, as the antennas become electrically larger. This monotonic behavior over the studied frequencies is expected, since even at the highest frequency (915 MHz), the antenna length 97 mm translates to around 0:3 , so that the antenna is still electrically short enough for current cancellation not to occur in the antenna structure, and on the other hand, for electrically small antennas the achievable radiation efficiency is proportional to the antenna size. The results shown in Fig. 8 also indicate that increasing the conductor thickness increases the radiation efficiency and that the effect of the substrate material on the radiation efficiency is minor. Further, according to the results in Figs. 7 and 8, both loss resistance and radiation efficiency are seen to increase as a function of frequency. Recalling the expression for the radiation efficiency in terms of loss and radiation resistance from (17), it is found that this can happen, if the radiation resistance increases faster as a function of frequency, than the loss resistance. The measured backscattered signal power as a function of transmitted power for the manufactured tags is shown in Fig. 9. The threshold power for each tag is the value at which the curves begin (about 6 dBm). Possible differences between individual IC chips and in the quality of their connection to the antenna structure are assumed to have negligible effect on the presented results. According to the measurement results shown in Fig. 9, the thickest tag backscatters the strongest signal, regardless of the transmitted power and the thinnest tag backscatters the weakest response. This agrees with the theory, as the backscattered signal is proportional to the square of the tag antenna gain. Gain, in turn, is a product of the directivity and the radiation efficiency of the antenna, and since at a given frequency the directivity is completely determined by the antenna structure, the increased conductor loss affects the backscattered signal power through the radiation efficiency of the tag antenna. The observed difference between the measured curves as a function of the transmitted power is caused by the power-dependent input impedance, and consequently, the



Fig. 9. Measured backscattered signal power at 866 MHz as a function of the transmitted power.

power-dependent modulation efficiency of the tag; the conductor loss in itself is not power dependent. At transmitted powers below 20 dBm, the difference between the backscattered signal power from the 13.9-m tag and 50.5-m tag remains approximately constant and is therefore expected to be due to the difference in radiation efficiency rather than change in the modulation efficiency. From the system point of view, this is still rather modest difference: presently, the passive RFID systems are limited by the forward link, i.e., at the lowest incident power that the IC remains operational, there is still a clear margin between the receiver sensitivity and backscattered signal power. From the tag measurement results presented, it can be concluded that by screen printing with conductive screen printing ink it is possible to fabricate tags where the conductive traces are narrow (less than 1 mm) (Fig. 2). Printing of narrow traces was slightly problematic using flexography and flexographic ink in [8] and gravure printing and rotogravure ink in [15]. In this study, this was not an issue because the amount of ink, which is transferred to the substrate during the printing process, is larger than with the aforementioned techniques. The effect of using less ink on tag antenna performance shows the same trend as the results presented in [8]. In [21], screen printed coil tag antennas were investigated and screen printed coil tags were shown to have read ranges comparable to the copper wire antennas. In [22], screen printed patch antennas were evaluated. It was discussed that silver-ink-based singlelayer antennas work well and provide 70%–80% of the reading range compared with copper solutions. However, the efficiency of a patch-type antenna is strongly dependent on the material in the dielectric layer and in the conductor layers. Paper substrate has high loss tangents which makes the direct integration of patch antennas on paper substrates difficult. Plastic substrates have much better performance. Nilsson et al. obtained 18% reading

range reduction for a silver ink patch antenna on a lowdensity polyethylene substrate compared with a high-cost patch made out of gold plated copper on a low-loss microwave substrate. This is a very promising result in the printed tags point of view [22]. Screen printing and other printing methods have also been used to print microstrip lines. In addition to singlelayer structures, printing of the ground plane increases losses compared with copper ground planes [23]. Still, in [24], insertion loss in screen-printed silver ink microstrip line with screen printed ground plane on a common FR-4 substrate was measured to be less than 0.1 dB/cm higher than for the etched lines at frequencies up to 5 GHz. This is low enough for many RF-circuit implementations [24].

V. CONCLUSION RFID antennas were produced by screen printing using silver ink as a conductor material. The thickness of the ink layer affects the functioning of the tag through the skin effect. In this paper, we investigated the impact of this on tag antenna performance by analytic means, full-wave electromagnetic simulations and measurements on the printed tag antennas with ink layer thickness varying from around one penetration depth to more than three penetration depths. The obtained results show that using conductor layer thicknesses over one penetration depth does yield significant improvement to the electrical performance of the tag, but reducing the ink layer thickness can be a favorable tradeoff to improve cost effectiveness through material savings. However, the thickness of the conducting layer affects the input impedance of the tag antenna and this needs to be taken into account in the tag antenna design. Further, we found that from the system point of view, the impact of the reducing the conductive layer thickness


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down to approximately one penetration depth has only modest impact on the reverse link performance. When printing is used to produce conductive patterns, the ink layer is not uniform. Thickness variations occur REFERENCES [1] G. Gustafsson, BDPPVNew method for patterning of laminated foils, Aperturen, vol. 2, pp. 19–20, 2009. [2] S. Merilampi, L. Ukkonen, L. Syda¨nheimo, P. Ruuskanen, and M. Kivikoski, BAnalysis of silver ink bow-tie RFID tag antennas printed on paper substrates,[ Int. J. Antennas Propag., vol. 2007, 2007, 9 p. DOI: 10.1155/2007/ 90762. [3] S. E. Molesa, BUltra-low-cost printed electronics, electrical engineering and computer sciences,[ Univ. California at Berkeley, Berkeley, CA, Tech. Rep. UCB/EECS-2006-55. [4] J. Costenoble, BRotary screen printing: The productive solution for HF/UHF RFID labels,[ SGIA J., pp. 7–10, 4th quarter, 2005. [5] M. Pudas, N. Halonen, P. Granat, and J. Va¨ha¨kangas, BGravure printing of conductive particulate polymer inks on flexible substrates,[ Progr. Organic Coatings, vol. 54, pp. 310–316, 2005. [6] A. Blayo and B. Pineaux, BPrinting processes and their potential for RFID printing,[ in Proc. Joint sOc- EUSAI Conf., Grenoble, France, Oct. 2005, pp. 27–30. [7] J. Siden, T. Olsson, M. Fein, A. Koptioug, and H.-E. Nilsson, BReduced amount of conductive ink with gridded printed antennas, polytronic 2005,[ in Proc. 5th Int. IEEE Conf. Polymers Adhesives Microelectron. Photon., 2005, pp. 86–89. [8] J. Siden, M. K. Fein, A. Koptyug, and H.-E. Nilsson, BPrinted antennas with variable conductive ink layer thickness,[ IET Microw. Antennas Propag., vol. 1, no. 2, pp. 401–407, 2007.

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substrates,[ Microelectron. Reliab., vol. 49, no. 7, pp. 782–790, Jul. 2009. S. Merilampi, V. Haukka, L. Ukkonen, P. Ruuskanen, L. Syda¨nheimo, M. Kivikoski, C.-H. Loo, F. Yang, and A. Elsherbeni, BPrinted RFID tag performance with different substrate materials,[ in Proc. 5th Int. New Exploratory Technol. Conf., Turku, Finland, Aug. 20–22, 2008, pp. 265–274, TUCS General publication 50. M. Ma¨ntysalo and P. Mansikkama¨ki, BAn inkjet-deposited antenna for 2.4 GHz applications,[ Int. J. Electron. Commun., vol. 63, no. 1, pp. 31–35, 2009. S. Y. Y. Leung and D. C. C. Lam, BGeometric and compaction dependence of printed polymer-based RFID tag antenna performance,[ IEEE Trans. Electron. Packag. Manuf., vol. 31, no. 2, pp. 120–125, Apr. 2008. H.-E. Nilsson, J. Siden, T. Olsson, P. Jonsson, and A. Koptioun, BEvaluation of a printed patch antenna for robust microwave RFID tags,[ IET Microw. Antennas Propag., vol. 1, no. 3, pp. 775–781, 2007. P. S. A Evans, B. J Ramsey, D. J. Harrison, and P. R. Shepherd, BFurther developments of microwave CLFs,[ in Proc. Autom. R.F. Microw. Meas. Soc. Conf., Malvern, U.K., Apr. 1997, 6 p. T. Bjo¨rninen, S. Merilampi, L. Ukkonen, P. Ruuskanen, and L. Syda¨nheimo, BPerformance comparison of silver ink and copper conductors for microwave applications,[ IET Microw. Antennas Propag., to be published.

ABOUT THE AUTHORS Sari Linnea Merilampi received the M.Sc. degree in electrical engineering from Tampere University of Technology (TUT), Pori, Finland, in December 2006, where she is currently working towards the Ph.D. degree. She is currently a Researcher at TUT, Pori. She has authored publications in the field of passive UHF RFID as well as materials and manufacturing methods of electronics. Her research interests are focused on printed electronics, passive UHF RFID, and electronics materials.

¨ rninen received the M.Sc. degree in Toni Bjo electrical engineering from Tampere University of Technology (TUT) in 2009, where he is currently working towards the Ph.D. degree in electrical engineering. Currently, he is a Researcher in the RFID research group at the Department of Electronics, TUT. His research interests include antenna technologies for RFID applications and modeling of electromagnetics. He has authored publications on passive RFID tags and application of printable electronics microwave applications.

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¨ ki was born in 1954, in Turku, Antti Vuorima Finland. He studied physics, theoretical physics, and mathematics at the University of Turku and received the B.Sc., M.Sc., Ph.L., and Ph.D. degrees from the Faculty of Mathematics and Natural Sciences in 1978, 1985, 1987, and 1989, respectively. Since 1992, he has been an Adjunct Professor in Physics at the University of Turku and since 2001 a Lecturer in Physics at the Pori Unit of the Tampere University of Technology. His main research interests are experimental and theoretical aspects of nuclear magnetic resonance, especially in solids at low temperatures.

Leena Ukkonen received the M.Sc. and Ph.D. degrees in electrical engineering from Tampere University of Technology (TUT) in 2003 and 2006, respectively. Currently, she is an Adjunct Professor and Head of RFID Research Group at the Department of Electronics, TUT. She has authored over 90 publications in the field of RFID antenna design and RFID applications. Her research interests are focused on passive UHF RFID antenna development for tags and readers.



Pekka Ruuskanen received the M.Sc. degree from Tampere University of Technology in 1978 and subsequently received the Ph.D. degree in technology in 1987 majoring in materials science and materials physics. He worked as a long term visiting staff member at the Los Alamos National Laboratory (1988– 1989). He worked as a Research Scientist at the Technical Research Centre of Finland (1990– 2003), Professor in Materials Science at the University of Vaasa (2003–2006), and Professor in Electronics at Tampere University of Technology (2006–present). His research interest is electronic materials science.

¨ nheimo received the M.Sc. and Ph.D. Lauri Syda degrees in electrical engineering from Tampere University of Technology (TUT). Currently, he is a Professor at the Department of Electronics, TUT, and works as a Research Director of Tampere University of Technology’s Rauma Research Unit. He has authored over 120 publications in the field of RFID tag and reader antenna design and RFID system performance improvement. His research interests are focused on wireless data communication and RFID.


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